Internal combustion engines, including diesel engines and spark ignition engines running on gasoline, produce small amounts of by-products in addition to the major products of combustion, carbon dioxide and water. The low level by-products include products of incomplete combustion such as carbon oxides (COx) and unburned hydrocarbons. In addition, low levels of nitrogen oxides (NOx) are generally produced during combustion of fuels when oxygen partially combines with nitrogen in the air. It is generally desirable to reduce the level of these by-products to improve the quality of the environment and to achieve greater fuel efficiency as a result of more complete combustion of the fuel.
Today it is conventional to remove nitrogen oxides and incomplete combustion products from the exhaust streams of an internal combustion engine by passing the exhaust stream through a so-called three-way catalyst. The three-way catalyst catalyzes the reduction of NOx to nitrogen, the oxidation of hydrocarbons to carbon oxides (COx), and the oxidation of COx to CO2. In many commercial embodiments, the three-way catalyst is based on a combination of platinum, rhodium, and palladium. In one mode of action, the presence of hydrocarbons and carbon oxides in the exhaust stream increases the efficiency of the reduction of NOx to nitrogen by the three-way catalyst.
It is also generally desirable to increase fuel efficiency in a total combustion engine by achieving more complete combustion of the fuel. More complete combustion of the fuel increases gas mileage and lowers the emission of unburned hydrocarbons into the atmosphere. In recent years, diesel engines and lean-burn gasoline engines have been developed to increase gas mileage. Lean burn engines work by burning fuel with air in excess of that required for stoichiometric combustion of the fuel. A desirable result is that the fuel is more completely burned, thereby increasing fuel economy.
Because of the higher efficiency of combustion of the fuel, the level of unburned hydrocarbons is reduced in the exhaust stream of diesel engines and gasoline engines running under lean burn conditions. As a result, the conventional three-way catalysts are less efficient at reducing nitrogen oxides, because there is less hydrocarbon and carbon oxide in the exhaust stream to aid in that reduction.
It would therefore be desirable to provide methods for enhancing the reduction of nitrogen oxides to nitrogen in a catalytic system, especially in engines with a reduced level of unburned hydrocarbons in the exhaust.